For me it comes down to a few things;

Character transformation - every character continues to transform to a point of mature growth. Think of each of the characters from the beginning of the film compared to the end.

Cinematography - the cinematography is this is film is by FAR the best in the trilogy.

Action Choreography - the action scenes (ESPECIALLY the duel) are handled expertly.

The Acting - everyone's performance was pushed up a level

The Universe - the audience got to delve into the SW Universe so much more in this one. We saw the Emperor, got a better understanding of Vader, saw unconventional ships (like the Slave 1), different factions (the bounty hunters), and more.

The Music - some of the most iconic musical cues in history come from this film (Imperial March)

The Bad-guys win - an unconventional ending

Pacing - the pacing is the biggest one (beside cinematography) in my opinion. It flows so much nicer. SW almost fell apart because of how poorly paced the film was. The entire movie was saved by the editor. ESB didn't have this problem.